This invention relates to solderable terminations in solid electrolyte capacitors.
Numerous termination systems are known wherein a cured resin having a matrix of metal particles included therein overlies an electrical contact surface region of an electrical component. For example, in my patent U.S. Pat. No. 4,017,773 issued Apr. 12, 1977, a silver loaded silicone resin is deposited over the graphite covered body of a solid electrolyte tantalum capacitor, to form an electrical and a mechanical connection between this body surface and a cathode lead wire. Also in the patent to Millard et al, U.S. Pat. No. 3,889,357 issued June 17, 1975, there is described a screen printed solid electrolyte capacitor wherein silver particles held in an acrylic binder overlie the graphite coated solid electrolyte to impart solderability to it. Further, in a patent application to Thompson et al, Ser. No. 666,767 filed Mar. 15, 1976, the end cap terminals for a solid electrolyte capacitor include a silver or copper loaded resin that may have an outer coating of silver or other metal that improves solderability. The solderable outer coating may be applied by plating or by using a silver paste that is subsequently cured. The above mentioned patents and patent application are assigned to the same assignee as the present invention.
Yet another known termination system has a thin-walled copper coating over the carbon layer of a solid electrolyte capacitor, which coating is deposited by an arc spray technique that at least partially melts the copper particles to effect a bond between each other and to the underlying conductive body surface.
The above noted termination systems suffer from certain disadvantages. For example, those which give the strongest adhesion and therefore most reliable contact are not readily solderable i.e. conductive epoxies and other systems where conductive particles are dispersed in a carrier resin. Other termination systems in which metal is exposed by solvent evaporation or in which metal is applied directly by flame spray have in general, a much weaker bond which may be further weakened during solder application by leaching and differential expansions. While copper and nickel are less susceptible to solder leaching than silver, the processes such as plating and metal spraying by which such metals have been conventionally applied tend to require special precautions against chemical contamination of the component body by plating baths, and in general close process controls leading to relatively complex and expensive manufacturing procedures.
The foregoing discussion is particularly relevant to the cathodic termination of a solid electrolytic capacitor where certain unique problems exist. Namely, the surface to be contacted is a graphite covered solid electrolyte such as manganese dioxide. This surface may be relatively small as for example in the type of capacitor described in U.S. Pat. No. 3,889,357 where only one surface of the capacitor may be conveniently contacted. In such a capacitor it is critically important that a strong bond be established between the body of the capacitor and the solder contact because unlike the three dimensional cathode surface of a standard rectangular or cylindrical body having an anode lead wire extending therefrom, the cathode termination of the screened pad capacitor cannot encompass and grip the body.
It is also desirable in some constructions to provide a solderable coating to the tantalum substrate surface opposite to that having a screen printed porous anode formed thereon, to provide a solderable anode termination.
An object of this invention is to provide a high bond strength conductive resin for the termination of a solid electrolyte capacitor and rendering the surface of such resin highly solderable.
A further object of this invention is to provide a relatively low cost solderable and reliable termination system for a solid electrolyte capacitor.